Tuesday, May 28, 2013

A recent paper published in the Proceedings of the National Academy of Sciences [PNAS] finds "robust geological evidence" supporting Svenmark's theory of cosmoclimatology, finding changes in solar activity affect cosmic rays, which in turn affect cloud formation and climate. The paper is the first to study the effect on geological timescales, although many peer-reviewed publications have studied the effect on short timescales ranging from hours to decades. Full paper available here

Abstract:

The climatic effects of cloud formation induced by galactic cosmic rays (CRs) has recently become a topic of much discussion. The CR–cloud connection suggests that variations in geomagnetic field intensity could change climate through modulation of Cosmic Ray flux. This hypothesis, however, is not well-tested using robust geological evidence. Here we present paleoclimate and paleoenvironment records of five interglacial periods that include two geomagnetic polarity reversals. Marine oxygen isotope stages 19 and 31 contain both anomalous cooling intervals during the sea-level highstands and the Matuyama–Brunhes and Lower Jaramillo reversals, respectively. This contrasts strongly with the typical interglacial climate that has the temperature maximum at the sea-level peak. The cooling occurred when the field intensity dropped to < 40% of its present value, for which we estimate > 40% increase in Cosmic Ray flux. The climate warmed rapidly when field intensity recovered. We suggest that geomagnetic field intensity can influence global climate through the modulation of Cosmic Ray flux.

Introduction:

One of the main goals of paleoclimatology is to reveal factors that control the Earth’s climate. Besides widely accepted climate drivers such as insolation and air–ocean circulation, the effect of clouds induced by galactic cosmic rays (CRs) has recently become a topic of much interest (e.g., 1, 2). Changes in CR flux may affect cloud cover (3, 4) through charge-related processes such as ion-induced nucleation (5) or its effect on the global electric circuit (6), which in turn affect cloud microphysics and formation/radiation of clouds. This would affect the global heat balance by increasing/decreasing albedo (7). Recent observations and modeling support the link between cloud cover and global temperature on decadal time scales (8). A cooling effect caused by the CR-induced clouds was also observed during the Southern Hemisphere Magnetic Anomaly (9), although the interval of observation was too short to be solid confirmation. Longer-term evidence is needed to confirm the CR flux–climate coupling.

An alternative way of testing the CR flux–climate coupling is to examine climate across a huge CR flux change in geological history. The geomagnetic field is a major factor controlling CR flux over longer time scales (10), and geomagnetic reversals are always accompanied by large decreases in field strength, which cause a large increase in CR flux. The Matuyama–Brunhes (MB) and Lower Jaramillo (LJ) geomagnetic polarity reversals provide suitable opportunities for such a test. During these reversals, the field intensity decreased to 10–20% of its present value for several thousand years (11). Additionally, the MB and LJ reversals occurred during interglacial periods, times when the impact of increased cloud cover on climate should be more readily detectable than in glacial periods. In this study, we compare detailed (ca. 200- to 2,000-y resolution) multiproxy climate analyses of five interglacial periods to see whether those with a geomagnetic reversal have unique features, and whether these features can be ascribed to field intensity variation. A pioneering study in this field examined the climatic effects of the extremely low geomagnetic field strength of the Laschamp excursion at ca. 40 ka and did not find an anomalous cool interval induced by a large increase in CR flux (12). The negative result may be due to the short duration of the Laschamp excursion and its occurrence during a glacial interval. The advantages of the present study, targeting the MB and LJ reversals are that i) the duration of extremely weak geomagnetic intensity spans roughly 5,000 y and ii) we search for cooling during warm climates near peak interglacials.